The war diaries of the poet Siegfried Sassoon have been made available online for the very first time.

Penned in the trenches of WWI and still bearing some of the mud of the Somme when they were uncovered, about 4,100 pages of the writer’s personal archive have been digitised to mark the centenary of the conflict.

The work of the Cambridge University Library, which aims to highlight the horrors of the First World War,the cache includes draft copies of Sassoon’s 1917 Soldier’s Declaration, his lone cri de coeur against the continuing madness of the fighting which established him as a focal point for dissent within the armed forces.

It went, “I am a soldier, convinced that I am acting on behalf of soldiers. I believe that this War, on which I entered as a war of defence and liberation, has now become a war of aggression and conquest” - in response to which the British Army committed Sassoon to a mental hospital.

Also featured are his poems, prose and sketches, including previously unpublished material and early drafts of some of his best-known works such as a The Dug-Out, which condemned the sending of young men such as himself to their almost certain doom and boasted the chilling hilling lines, “You are too young to fall asleep forever, and when you sleep you remind me of the dead.”

Sassoon’s thoughts and ideas were scribbled into notebooks small enough to fit in the pocket of his army tunic, and his words and drawings starkly captured the daily hell of those hunkered down deep in the field of battle, be it listing the members of his battalion and their fates, making notes on military briefings or drawing diagrams of the trenches.

“The war diaries Sassoon kept on the Western Front and in Palestine are of the greatest significance, both nationally and internationally,” said Cambridge University librarian Anne Jarvis.

“We are honoured to be able to make them available to everyone, anywhere in the world, on the 100th anniversary of WWI.

“From his Soldier’s Declaration to his eyewitness accounts of the first day of battle on the Somme, the Sassoon archive is a collection of towering importance, not just to historians, but to anyone seeking to understand the horror, bravery and futility of the fighting, as experienced by those on the front lines and in the trenches.”

Made up of 23 journals covering the periods 1915-27 and 1931-32, the online archive also includes two poetry notebooks from 1916-18 and has largely been out of reach of the public and researchers until now, due to the poor physical condition of the documents.

Indeed, only Sassoon’s official biographer, Max Egremont, has had access before today.

In the journals, Kent-born Sassoon, who served with the Royal Welch Fusiliers, describes the first day of the Somme as a ‘sunlit picture of hell’ and the diaries also record the moment he was shot by a sniper at the Battle of Arras, as well as a psychological profile of ‘the soul of an officer’.

“The great array of activities, difficulties and dangers that faced him as a serving officer, and the recurring inspiration of his creative responses to his conditions, are represented in the range of uses to which he put these notebooks,” said the library’s John Wells.

“Unlike edited, printed transcriptions, the digitisations allow the viewer to form a sense of the physical documents, and to appreciate their unique nature as historical artefacts.”